The present invention relates to an organic light emitting device, and in particular, to a drive scheme for an organic light emitting device.
Light emitting devices are becoming more popular as an image source in both direct view and virtual image displays. The popularity is due, at least in part, to the potential of generating relatively high luminance at relatively low power levels. For example, reflective liquid crystal displays can only be used in high ambient light conditions because they derive their light from the ambient light. Also, liquid crystal displays with back lights may be used in low ambient light conditions because they primarily derive their light from the back light. However, such liquid crystal displays are generally too large for practical use in very small devices.
Organic light emitting devices are especially suitable for use in very small devices, such as pagers, cellular and portable telephones, two-way radios, data banks, radios, etc. Organic light emitting devices are capable of generating sufficient light for use in displays under a variety of ambient light conditions, from no ambient light to high ambient light. Also, organic light emitting devices can be fabricated relatively cheaply and in a variety of sizes from very small (less than a tenth of a millimeter in diameter) to relatively large. In addition, light emitting devices have the added advantage that their emissive operation provides a very wide viewing angle.
Generally, organic light emitting devices include a first electrically conductive layer (or first contact), an electron transporting and emission layer, a hole transporting layer, and a second electrically conductive layer (or second contact). The light can be transmitted either way but typically exits through one of the conductive layers. There are many ways to modify one of the conductive layers for the emission of light there-through but it has been found generally that the most efficient light emitting device includes one conductive layer which is transparent to the light being emitted. Also, one of the most widely used conductive, transparent materials is indium-tin-oxide (ITO), which is generally deposited in a layer on a transparent substrate such as a glass plate.
Referring to FIG. 1, a conventional driving system for driving a luminous element is shown. The driving system shown in FIG. 1 is generally referred to as a simple matrix driving system in which anode lines A1 through Am and cathode lines B1 through Bn are arranged in a matrix (grid). In the driving system shown in FIG. 1 luminous elements E1,1 through Em,n are connected at each intersection of the anode lines and cathode lines. The driving system causes the luminous element at an arbitrary intersection to emit light by selecting and scanning one of the anode lines and the cathode lines sequentially at fixed time intervals and by driving the other of the anode and cathode lines by current sources 521 through 52m, i.e., driving sources in synchronism with the scan.
Thus, there are traditionally two systems for driving luminous elements by means of the driving sources: (1) a system of scanning the cathode lines and driving the anode lines, and (2) a system of scanning the anode lines and driving the cathode lines. FIG. 1 illustrates the former case of scanning the cathode lines and driving the anode lines.
As shown in FIG. 1, the cathode line scanning circuit 51 is connected to the cathode lines B1 through Bn and the anode line driving circuit 52 comprising the current sources 521 through 52m is connected to the anode lines A1 through Am. The cathode line scanning circuit 51 applies a ground potential (0 volts) sequentially to the cathode lines B1 through Bn by scanning these lines while switching switches 531 through 53n to the side of a ground terminal at fixed time intervals. The anode line driving circuit 52 connects the current sources 521 through 52m with the anode lines A1 through Am by controlling ON/OFF of switches 541 through 54m in synchronism with the scanning of the switches of the cathode line scanning circuit 51 to supply driving current to the luminous element at the desired intersection. In essence, a potential is imposed across or a current passed through the light emitting material.
When the luminous elements E2,1 and E3,1 are to emit light, for example, the switches 542 and 543 of the anode line driving circuit 52 are switched to the side of the current sources to connect the anode lines A2 and A3 with the current sources 522 and 523. At the same time the switch 531 of the cathode lines scanning circuit 51 is switched to the ground side so that the ground potential is applied to the first anode line B1. The luminous elements are controlled so that the luminous element at an arbitrary position emits light and so that each luminous element appears to emit light concurrently by quickly repeating such scan and drive.
A reverse bias voltage Vcc, which is equal to the source voltage, is applied to each of the cathode lines B2 through Bn. The reverse bias voltage Vcc is not applied to the cathode line B1 being scanned in order to prevent erroneous emission. It should be noted that although the current sources 521 through 52m are used as the driving sources in FIG. 1, the same effect may be realized also by using voltage sources.
Each of the luminous elements E1,1 through Em,n connected at each intersection may be represented by a luminous element E having a diode characteristic and a parasitic capacitor C connected in parallel, as shown by the equivalent circuit in FIG. 2. Traditional driving systems described above have had problems due to the parasitic capacitor C within the equivalent circuit. The problems are described as follows.
FIGS. 3A and 3B illustrate each of the luminous elements E1,1 through E1,n using only the parasitic capacitors C described above by excerpting the part of the luminous elements E1,1 through E1,n connected to the anode line A1 in FIG. 1. When the cathode line B1 is scanned and the anode line A1 is not driven, the parasitic capacitors C1,2 through C1,n of the other luminous elements E1,2 through E1,n (except the parasitic capacitor C1,1 of the luminous element E1,1 connected to the cathode line B1 currently being scanned), are charged by the reverse bias voltage Vcc applied to each of the cathode lines B1 through Bn, in the direction as shown in FIG. 3A.
Next, when the scanning position is shifted from the cathode line B1 to the next cathode line B2 and the anode line A1 is driven in order to cause the luminous element E1,2 to emit light, for example, the state of the circuit is shown in FIG. 3B. Thus, not only is the parasitic capacitor C1,2 of the luminous element E1,2, which emits light changed, but the parasitic capacitors C1,1 and C1,3 through C1,n of the luminous elements E1,1 and E1,3 through E1,n connected to the other cathode lines B1 and B3 through Bn, also are charged because currents flow into the capacitors in the direction as indicated by arrows.
In general, luminous elements can not emit light normally unless a voltage between both ends thereof builds up to a level which exceeds a specified value. In the traditional driving system, not only is the parasitic capacitor C1,2 changed when E1,2 is to emit light, but the parasitic capacitors C1,3 through C1,n of the other luminous elements E1,3 through E1,n are charged as well. As a result, the end-to-end voltage of the luminous element E1,2 connected to the cathode line B2 can not build up above the specified value until the charging of all of these parasitic capacitors of the luminous elements is completed.
Accordingly, such a system has the limitation that the build up speed until emission is slow. Also no fast scan can be attained due to the parasitic capacitors described above. Further, because the parasitic capacitors of all the luminous elements connected to the anode line have to be charged, the current capacity of the driving source for driving the luminous elements connected to each anode line must be large. The aforementioned problems become more significant as the number of luminous elements increase.
Okuda et al., U.S. Pat. No. 5,844,368, disclose an improved driving system for an organic light emitting device in which all cathode lines and all anode lines are reset by dropping their voltage to a ground potential once in a shifting scan to the next cathode line. Similarly, Okuda et al. likewise disclose a driving system that corresponds to a case when all of the cathode lines and anode lines are reset once to the source voltage Vcc before the next cathode line is scanned. Further, Okuda et al. disclose a driving system that corresponds to a case when all of the cathode lines are reset to Vcc and the anode lines are preset, in order to be ready for the next emission before the next cathode line is scanned.